1. Field of the Invention
The invention pertains to the field of molecular genetics and, in particular, to the use of man-made nucleotides in diagnosing animal diseases or vaccinating animals against disease. More specifically, the preferred nucleotides derive from an immunologically distinct strain of the porcine reproductive and respiratory syndrome ("PRRS") virus, and selectively target this virus in the application of vaccination or diagnostic techniques.
2. Description of the Prior Art
A new viral disease of pigs was detected in North America during 1987, and reported by Hill, Overview and History of Mystery Swine Disease (Swine Infertility and Respiratory syndrome), in Proceedings of the Mystery Swine Disease Committee Meeting, October 6, Denver Colo., from the Livestock Conservation Institute of Madison, Wis. pp. 29-30 (1990). A disease having substantially identical clinical signs was found in Europe during 1990, as reported by Paton et al., Blue ear disease of pigs, 128 Vet Rec. 617 (1991). The clinically observed disease is commonly known by various names including porcine reproductive and respiratory syndrome ("PRRS"), swine infertility and respiratory syndrome ("SIRS"), porcine epidemic abortion and respiratory syndrome ("PEARS"), and mystery swine disease; herein, the term PRRS will suffice to indicate all of these names.
The consequences of this disease included late-term abortions and stillbirths in sows, as well as respiratory insufficiencies in nursery pigs that developed poorly and died easily. Decreases were observed in sow conception rates and litter sizes. Estimates stated that about ten to fifteen percent of pig production were lost annually due to reproductive failure. Early clinical signs of the disease included anorexia and mild pyrexia. Other signs included bluish discolorations on the skin of diseased herd animals, with the discolorations being primarily located on the ears, teats, snout, and frontal portions of the neck and shoulders. Necropsy results indicated thickened alveolar septae caused by the presence of macrophages, degenerating cells, and debris in alveolar spaces. These abnormalities indicated the presence of PRRS virus.
The causative viral agent was suspected to be a small, enveloped positive-stranded RNA virus that was recovered primarily from alveolar macrophages of infected swine, as reported by Benfield et al., Characterization of swine infertility and respiratory syndrome (SIRS) virus (isolate ATCC VR-2332), 4 J. Vet. Diagn. Invest. 127-133 (1992); and Wensvoort et al., Lelystad virus, the cause of porcine epidemic abortion and respiratory syndrome: a review of mystery swine disease research at Lelystad, 33 Vet. Micro. 185-193 (1992). The isolation technique for the Lelystad ("LV") virus included homogenizing infected swine lung tissue; mixing the homogenate with a physiological saline, e.g., Ringers solution, Hank's balanced salt solution, and Minimum Essential Medium ("MEM") to a 10% weight/volume amount of the homogenate; and filtering the mixture through a series of 0.45, 0.2 and 0.1 micron filters.
The LV virus appeared to be closely related to arteriviruses in morphology, genome organization, transcriptional regulation, and macrophage specificity, according to Plagemann et al., Lactate dehydrogenase-elevating virus, equine arteritis virus and simian hemorrhagic fever virus: a new group of positive-strand RNA viruses, 41 Adv. Vir. Res. 99-192 (1992).
The complete nucleotide sequence of the LV strain of the PRRS virus was identified by Meulenberg et al., Lelystad virus, the causative agent of porcine epidemic abortion and respiratory syndrome (PEARS), is related to LDV and EAV, 192 Virology 62-72 (1993). A partial LV sequence was also identified by Conzelmann et al., Molecular characterization of porcine reproductive and respiratory syndrome virus, a member of the arterivirus group, 193 Virology 193, 329-339. The positive-strand genome of the LV virus (Sequence ID. Nos. 14-26) included eight open reading frames ("ORFs"), which had some similarity in comparison with the genes of coronaviruses and arteriviruses. Two open reading frames likely coded for the viral RNA polymerase. LV ORFs two through six appeared to code for structural proteins associated with viral membranes, and LV ORF 7 was believed to code for a nucleocapsid.
The LV viral proteins were expressed from a nested set of RNA transcripts that had overlapping 3' ends. While this expression strategy was shared with the Coronavirus family, the physical properties of the LV virus originally placed it in the Togavirus family. Plagemann et al. (see above) has proposed a new family, the Arteriviridae, to encompass viruses having these dual properties. This family included the PRRS virus, equine arteritis virus ("EAV"), lactate dehydrogenase-elevating virus ("LDV"), and simian hemorrhagic fever virus ("SHFV").
A second strain ("VR-2332") of the PRRS virus was isolated as a fourth cell culture passage, as reported by Benfield et al., Characterization of swine infertility and respiratory syndrome (SIRS) virus (isolate ATCC VR-2332), 4 J. Vet. Diagn. Invest. 4, 127-133 (1992). Nevertheless, the viral genome was not sequenced. The VR2332 isolate was deposited in the American Type Culture Collection, and now has an ATCC catalogue number VR-2332. The VR-2332 virus was characterized as spherical with an average diameter of 62 nm and a 25-30 nm core surrounded by an envelope. Viral particles had a buoyant density of 1.18-1.19 g/ml in cesium chloride and were further purified from filtered tissue homogenates by centrifugation on cesium chloride gradients.
The respective VR-2332 and LV virus isolates displayed vast differences in terms of antigenic variation, especially in view of their common morphology and similar clinical signs in swine. A comparison study between 24 field sera and seven viral isolates from Europe and North America failed to distinguish a single common antigen which was able to diagnose infection in a reliable manner for both viruses, as reported by Wensvoort et al., Antigenic comparison of Lelystad virus and swine infertility and respiratory syndrome (SIRS) virus, 4 J. Vet. Diagn. Invest. 134-138 (1992). Accordingly, despite the structural and symptomological similarities between the two virus strains, it is unlikely that a single vaccine could be developed from one strain of the virus for purposes of immunizing swine against both strains.